Accurate measurement and adjustment of the properties of paper pulp are required in the manufacture of good-quality paper. In the drainability measurement (such as freeness or Schopper-Riegler value) of paper pulp, the speed at which the paper pulp can be separated from water is determined empirically. Drainability depends on a plurality of factors, such as fibres, pulp processing (e.g. mechanical/chemical), amount of fine substance, temperature, consistency and measuring device.
One of the most known drainability measurement methods is CSF (Canadian Standard Freeness). This measurement method is a standard and it is presented in more detail in publication T 227 om-99, Freeness of pulp, TAPPI, 1999. At the beginning of the CSF drainability measurement, exactly one liter of sample is taken into a measurement chamber. The lower lid is opened, and the sample is allowed to settle in the reservoir, whereby some pulp is descended onto a wire at the lower part of the reservoir. When 5 seconds have lapsed from the opening of the lower lid, the air valve is opened, whereby water starts to be discharged from the pulp sample through the wire and the pulp accumulating on the wire into a funnel. Part of the water flows out of a lower branch of the funnel, which is thought to release water as a predetermined flow. Water accumulating over a predetermined constant volume, in turn, flows out of a side branch if the flow of liquid flowing into the funnel exceeds a predetermined flow. In drainability measurement, it is exactly the amount of water flown from the side branch that is measured with a measuring glass, and this amount of water corresponds to the drainability, the CSF value. The measurement is usually performed manually. The measurement is cumbersome and sensitive to changes in both consistency and temperature. In addition, at low CSF values, the measurement is inaccurate.
Another known drainability determination method is the so-called Schopper-Riegler method, which is presented in publication SCAN-C 19:65, Scandinavian pulp, paper and board, Testing committee, approved in 1964. This measurement, too, is based on the flow of water flowing through the wire through the lower branch and the side branch, and the water flown through the side branch determines the drainability. This measurement, too, is usually performed manually. As the previous measurement, this measurement, too, is cumbersome and sensitive to changes in both consistency and temperature. In addition, at extreme values, in particular, the SR measurement is inaccurate.
In addition, Finnish patent 104855 discloses a solution, wherein the drainability can be determined computationally by measuring the flow from the measurement chamber, by determining the point in time when the flow from the measurement chamber corresponds to a predetermined flow, and by establishing the drainability as a function of the amount flown from the measurement chamber up to a determined point in time. However, since this solution, too, is associated with inaccuracy, there exists a need for a more accurate drainability measurement than the previous ones.